The "casting time" of using a potion/ether, and the period afterwards where you can't do anything but auto-attack would make them ineffective for carrying stacks for the purpose of replacing a whm. Find yourself at 300 hp and wanna pop something that gives half your hp back, fine, but if every did did this every time on virtually any relevant nm (anything in even Skirmish), the fights would be much longer.
As most nm's become more and more resistant to stun, and there's usually a hard-limit on how long you can fight, no ls would want their DDs spamming meds. DDs don't go through stacks of remedies to get rid of slow, even with as busy as the wm might be.
And besides, to say that more useful potions is more important than stacking is ignoring the fact that a lot of players carry obscene amounts of gear around for their job. I usually ahve 3-4 stacks of echos and remedies in accessible inv, but I keep 2 stacks of each in my main inventory on my pld.
Stacking to 99 would give me 4 more inventory spaces and a common place to put them.
Lastly, on most servers, you're silly to buy echos/remedies off the AH (sorry alchemists) there's usually a vendor conveniently located that sells for cheaper.